1. Field of the Invention
The present invention relates to wide band frequency amplifiers and, more particularly, to wide band frequency amplifiers that admit input signals of large amplitudes on the high frequency band.
2. Description of the Prior Art
Heretofore, push-pull amplifier circuits of the type shown in FIG. 1 have been widely used as an output stage circuit. The prior art output stage circuit such as this one typically comprises an NPN type transistor Q1 and a PNP type transistor Q2 connected via their emitters. The transistors Q1 and Q2 act alternately in response to a leading edge and a trailing edge of an input signal V.sub.IN.
A bias voltage source V1 for the transistors Q1 and Q2 is applied between the bases thereof. In addition, the base of the PNP type transistor Q2 is connected to a current source 2, the other end of the current source 2 being connected to ground. The transistor Q1 is coupled with a transistor Q in Darlington connection. The collector of the transistor Q1 is connected to a reference voltage source Vcc. The base of the transistor Q3 is fed with the input signal V.sub.IN from a signal source 3 via a signal source resistor R1.
The rising characteristic of the output stage circuit 1 given the input signal V.sub.IN is high thanks to the transistors Q1 and Q3 being held in Darlington connection. The falling characteristic of the output stage circuit 1 is determined by the amount of an emitter current that flows through the PNP type transistor Q2. This is because the terminal capacitor C1 for pins or other elements connected to the output end of the circuit must be driven by that emitter current.
The time t.sub.f required for an output signal V.sub.OUT to fall (see FIG. 2) is represented by the equation: ##EQU1## where, A stands for the amplitude of the input signal (Vpp), I for the current that flows through the current source 2, h.sub.FE for the current amplification factor of the transistor Q2, and C for the capacitance of the terminal capacitor C1.
In the setup above, the cut-off frequency f.sub.max of the frequency characteristic f decreases even if the input amplitude A becomes smaller or if the current amplification factor h.sub.FE is lowered, as indicated by the equation: ##EQU2##
Given this characteristic, to widen the frequency band requires boosting the current I that flows through the current source 2 or enlarging the current amplification factor h.sub.FE.
The current source 2 is constituted by an NPN transistor Q4 having a fixed bias voltage source connected thereto, as shown in FIG. 3. Thus increasing the current I reduces the current amplification factor h.sub.FE of the transistor Q4. This lowers the base voltage and causes a negative feedback, putting constraints on the maximum current that may flow through the current source 2.
One way to bypass this drawback is to increase the maximum current by widening the cell area of the NPN type transistor Q4. But this would raise the parasitic capacitances C.sub.CS and C.sub.CB of the transistor Q4. With the cell area of the PNP transistor Q1 also needed to be widened correspondingly, the parasitic capacitance C.sub.CB of the transistor Q1 would also be raised.
Because simply boosting the amount of the current flowing through the current source 2 would entail increases in parasitic capacitance, an additional current required to drive that capacitance hampers the attempt to expand the frequency band as desired. Furthermore, in a situation where the current amount is raised while the input signal V.sub.IN is withheld, the current I that flows through the current source 2 flows into the transistor Q3 as an emitter current. This requires furnishing an emitter follower amplifier circuit upstream for driving the transistor Q3. That in turn restricts the dynamic range of the output signal V.sub.OUT.
On the other hand, attempts to raise the current amplification factor h.sub.FE are hampered by the fact that the higher the frequency band, the smaller the current amplification factor h.sub.FE of the PNP type transistor Q2. Thus the cut-off frequency f.sub.max may not be raised. For example, input signals of large amplitudes permit band expansion of only a third to half of the transition frequency f.sub.T. The degraded frequency characteristic on the high frequency band must be compensated by getting large currents to flow.
Another way to improve the frequency characteristic is to have a PNP type transistor Q7 coupled with a transistor Q5 in Darlington connection, the base of the transistor Q7 being connected to a connective midpoint between a current source 4 and a bias voltage source V3, as shown in FIG. 4. In this case, the minimum value of the output signal V.sub.OUT fails to become lower than a level of 2 Vbe+Vsat, where Vsat represents the voltage of an NPN type transistor Q8 constituting part of the current source 4. As a result, the dynamic range is narrowed.
Prior art push-pull circuits of the above-described construction have a fixed value of the current I flowing through the constant current source 2 or 4. This arrangement is intended to have the transistor Q2 or Q5 drive the load capacitance C at a desired frequency.
It should be noted here that the higher the frequency, the smaller the current amplification factor h.sub.FE for the transistor Q2 or Q5. This is because the transition frequency f.sub.T of PNP type transistors is lower than that of NPN type transistors. Given the characteristic, extending the frequency characteristic f up to the desired level requires raising the current I of the constant current source 2 or 4.
Meanwhile, with the input signal withheld, the emitter current of the transistor Q3 or Q6 becomes the constant current I. Raising the current I promotes a voltage drop I.sub.0 /{h.sub.FEN1 +1} resulting from the combination of the input impedance R.sub.L of the transistor Q3 or Q6 and the base current flowing therethrough (h.sub.FEN1 : current amplification factor of transistor Q3 or Q6). This in turn narrows the dynamic range.
One solution to the above drawback is to provide another transistor for driving the transistor Q3 or Q6. However, this arrangement also reduces the dynamic range for the reason described. Given the above constraints, raising the current I has not been an easy task.